Hydrodynamic interactions in tandem flapping wing systems

We review recent studies on the hydrodynamic interactions in tandem flapping wing systems, with a focus on the interplay between vortex dynamics and structural flexibility. We examine configurations including rigid wings with prescribed motion, spanwise-flexible wings, and chordwise-flexible wings. For rigid wings, three-dimensional effects are shown to reduce thrust, but they can be mitigated through aspect ratio tuning. Flexibility introduces fluid-structure resonance phenomena that enhance aerodynamic performance. In tandem systems, performance optimization requires careful coordination: forewings benefit from resonance, while hindwings perform best with slightly stiffer configurations to effectively capture upstream wake structures. Finally, in systems of self-propelled flexible flappers, hydrodynamic interactions give rise to coordinated motion and energy savings through flow-mediated feedback.